(USA TODAY) -- Concerns about safety and side effects for the human papillomavirus (HPV) vaccine - one of the newest shots recommended for adolescents - has increased among parents: 16% cited these fears as the main reason they did not have their daughters vaccinated in 2010, up from 5% in 2008, a new study finds.

And the percentage of parents who said they did not intend to vaccinate their daughters against HPV in the next 12 months also grew, from 40% in 2008 to 41% in 2009 to 44% in 2010, even as parents reported increasing physician recommendations to get the shot, says the study in April's Pediatrics, released online Monday.

(Although now recommended for both girls and boys at age 11 or 12, the HPV vaccine was not widely prescribed for males at the time.)

HPV, a family of sexually transmitted viruses linked to tumors of the cervix, head and neck and several organs, is passed on through genital contact, most often during vaginal and anal sex, but can also be passed on during oral sex.

According to the Centers for Disease Control and Prevention, HPV vaccines offer the best protection to girls and boys who receive all three vaccine doses and have time to develop an immune response before being sexually active with another person.

Parental anxiety over the safety of other new adolescent vaccines - the tetanus, diphtheria and pertussis booster Tdap, (and a version called Td without the pertussis portion) and the meningococcal disease shot MCV4 - did not show a similar increase.

While the up-to-date immunization rates rose for all those vaccines, the proportion of girls fully immunized against HPV (three doses over six months) was substantially lower than the proportion for the other two vaccines.

"You'd expect as people get more familiar with a vaccine that they would actually become more comfortable with it," says Paul Darden, professor of pediatrics at the University of Oklahoma Health Sciences Center and co-author of the study. "That doesn't seem to be the case with HPV," he says.

The findings suggests that additional approaches - from detailed discussions about vaccine safety and effectiveness between parents and doctors to additional "social media sorts of things" - may be needed to improve HPV immunization rates, says Darden.

He and colleagues analyzed data from the parents' report portion of the 2008-2010 National Immunization Survey of Teens, a government phone survey of households with adolescents ages 13 to 17.

It found that as of 2010, 81% of teens had the Tdap/Td vaccines and 63% had the MCV4 vaccine. Only 32% of girls were immunized against HPV.

More recent vaccination data from 2011, released after the new analysis, shows 85% of teens had the Tdap/Td vaccines; 71% had the MCV4 vaccine; and 35% of male and female teens were immunized against HPV.

The most frequent reasons given by parents for not vaccinating their teens with Tdap/Td and MCV4 in 2010 were similar and consistent over the three years but were more diverse and changed significantly when it came to the HPV immunization: 9% said it was not recommended; 17% said it was not needed/not necessary; 16% cited safety concerns/side effects; 11% said it was because their daughter was not sexually active.

(A study co-author has done work for Merck, which makes an HPV and Td vaccine, but the study was funded through a grant from the Department of Health and Human Services.)

Although lack of sexual activity declined as a key reason for parents' rejection of the vaccine (down from 14% in 2008), it "again points to parental misunderstanding about HPV vaccination" and suggests that more work is needed to help some parents understand the vaccination, says Gregory Zimet, professor of pediatrics and clinical psychology at Indiana University School of Medicine. He was not involved in the study

"It's particularly concerning that parental worries about safety have increased, given that evidence for the safety of HPV vaccination has increased over the same time period," says Zimet. "In fact, the evidence is overwhelmingly persuasive that HPV vaccines are quite safe

The first HPV vaccine was licensed in 2006, and even then "there was very strong evidence that it was safe," says Jessica Kahn, professor of pediatrics at Cincinnati Children's Hospital Medical Center. She was not involved in the study. That evidence "only continues to increase as tens of millions of doses have been administered and no evidence of safety concerns has emerged," says Kahn.